To make these creamy-inside, crisp-outside potatoes, we first boil whole fingerlings or small Yukon Golds, then flatten and roast them in a ripping-hot oven. We took a cue from Mokonuts, a popular Parisian café, and dressed the smashed potatoes with tangy-spicy vinaigrette that nicely accents the potatoes’ starchy, mildly sweet flavor. When boiling the potatoes, begin timing as soon as they’re added to the water. To make ahead, the potatoes can be boiled, smashed, cooled and refrigerated a day in advance; to finish, brush with oil and roast as directed. The vinaigrette can be made in advance except for the chilies, then covered and refrigerated until ready to use; bring to room temperature and add the chilies.